A radar transmitter is required to provide high reproducibility of a transmitted waveform so that modifications of the echoes received, in terms of phase and amplitude, can be evaluated to ascertain characteristics of the target. To maintain the square wave amplitude and phase of the transmitted waveform constant in time requires high short term stability of the modulated signal, of the radio frequency source and the final amplification stage.
In general the power amplifier which is used is a cathode modulated power amplifier which has a video modulating pulse which is a function of time. For given power amplifier characteristics the stability transmitting system is greater when the pulse to pulse stability of the modulating signal V.sub.K (t) is stable.
At the receiver the processing system is usually of the MTI/MTD type so as to suppress fixed echoes and detect transient echoes characterized by Doppler frequencies different from zero.
Such systems require that the transmission rate vary from pulse to pulse or from pulse group to pulse group to an amount which, peak-to-peak may reach up to 20% of the mean value.
With very sophisticated modulators, transmission pulse instability can be limited to about 0.1% with acceptable circuit complications, greater stability generally involving unacceptable circuit complications.
Through the use of good power transmitting tubes and modulators with these characteristics, it is possible to assure a degree of stability of the characteristics of the RF pulses transmitted so as to achieve typically the capability of suppressing fixed echoes at about 40 dB.
This value is about 10 dB less than what can be achieved if greater stability can be ensured with other elements of the transmitting and receiving chains.